If the year is before 0000 or after 9999, the following additional rules apply:
- must prefix any year before 0000
+ must prefix any year after 9999
The year must be separated from the next component with the following characters:
- if the next component is month or day of the year
Either - or W if the next component is week of the year
Q if the next component is quarter of the year
If the year component is prefixed with either - or +, and is separated from the next componen

So using / as the delimiter within the date component is not allowed

in the end provided you get the correct format you should then be able to run

match(n:MyNode)
where date(datetime(n:timestamp_prop))='2019-09-04' or
date(datetime(n:timestamp_prop))< 12019-09-05`;

such that the change is that the where clause previously referred to n:timestamp when it should have be n.timestamp (presuming the name of your property is named timestamp), and again enclosing 2019-09-05 in single quotes.